(1) Field of the Invention
This invention relates to mobile telecommunications and more particularly to communications which are automatically relayed to mobile stations from ground station to ground station according to the cellular system. Also specifically all systems described are suitable for aircraft.
(2) Description of the Related Art
This invention is described in relation to North American cellular systems. wherever, its teachings are equally applicable to other cellular systems such as GSM, NMT, and others used Worldwide.
North American cellular radio telephone service, primarily for automobiles or other land vehicles, currently uses a designated plurality, of radio channels namely a set of 832 radio channels in the Ultra High Frequency (UHF) radio band. As used here, "channel" means a pair of Ultra High Frequencies in the designated band. One frequency of the pair in a channel is called the "forward" carrier used for transmission from the base to the mobile, and the other frequency of the pair in the channel is used for transmission in the "reverse" direction from the mobile to the base. Two competitive operators in each service area each use 416 channels. Most present technology utilizes analog Frequency Modulation (FM radio) as the method for transmitting the speech, with a 30 kHz frequency channel spacing.
Cellular systems allow more conversations simultaneously in the same service area (e.g., city) than there are channels, because the radio channels are "re-used" at several different base radio locations in the overall service area. The cellular system divides up the service area into a number of cells. Each cell includes a base transmission station or tower. The radius of the cell will be basically the distance from the base tower that good reception is assured between the tower and the mobile station. Therefore it may be seen that if the entire area is covered numerous cells are required. The arrangement of the cells is often considered to be somewhat like hexagonal tiles covering the entire service area, however, it will also be understood that there is a border area between two cells where transmission and reception are about equal, or if not equal, are acceptable from either of two or three different base stations (sometimes called towers herein). The cell diameter would be approximately twice the cell radius which would be the distance of dependable transmission and reception. A widely used typical layout includes seven "cells," wherein a fraction of the total radio channel numbers can be used in each cell. Other arrangements are also used, but this discussion will describe the seven cell as a specific example.
In a completely set-up system, there are 416 radio channels available for each of the two competing operators in a service area (e.g., in Dallas-Fort Worth, Metrocell has 416 channels and Southwestern Bell Mobile Systems has another 416). Of these 416, 21 are reserved for sending call processing messages (or control) only and are not used for voice. The remaining 395 channels are divided into seven groups of approximately 56 channels each when used with a seven cell layout plan.
It will be understood that with a group of seven cells, any particular tower would be less than three radii in distance from about 2/3 of the mobile stations in other cells, and therefore potentially interfering. However, it will also be understood that with seven cells there would be one center cell with six other cells grouped around it. If additional cells were added it will be understood that mobile stations in each additional cell could be spaced at least more than five radii distant from any cell base station using the same channel; and therefore in a location where the potentially interfering signal strength would be very weak. Therefore the additional cell base stations could use the same radio channel as the cell whose border was more than five radii away.
Each cell represents the coverage area of a different base station antenna, based on radio signal power compared to interference from other base stations in the city using the same radio channels. For FM radio with 30 kHz band-width, it is known from extensive testing that the effective operational boundary of a cell is determined by the approximately circular boundary where the desired radio signal is stronger than the interference from other cells by a ratio of approximately 64 to 1. In radio jargon, this ratio is usually expressed by means of the logarithm of the power ratio, and is thus 18 decibels (dB). Real cells are often far from circular in shape due to irregularities in terrain, the effect of buildings and trees, etc. Other arrangements of the radio channels, using directional antennas rather than the omni-directional (circular pattern) cells are also used in some areas with very high subscriber densities. In the outermost cells of a service area, where there are no other cells beyond the edge of the service area, the outer perimeter of an individual cell is determined by the locations where the signal strength is 18 dB stronger than the interfering radio "noise" level in the receiver due to irreducible physical and device electrical random "noise."
The actual diameter of the cells in real systems, depending upon antenna height and base station radio transmitter power, varies by design from as little as about 5 km (3 mi) to as much as about 40 km (25 mi). The objective when designing an effective cellular system is to ensure that interfering radio signals on the same radio frequency throughout each cell remain weaker than the desired signal by this ratio of 18 dB.
Each cell contains a radio channel for control messages (called a setup or a control channel) in addition to the voice conversation channels referred to above. The control channels are generally taken from the pool of 21 channels legally reserved for that purpose. When a call is initiated, a sequence of special digitally coded messages are exchanged between the base and mobile radios on the control channel to locate and identify the mobile subscriber, and to determine what telephone number the mobile subscriber dialed when the call is originated by the mobile subscriber. The last message of the sequence directs the mobile radio to re-tune to an available voice channel for the purpose of conversation. During the conversation, the voice channel may be very briefly interrupted to transmit messages between the mobile and the base. These brief interruptions (typically only two tenths of a second and typically called "packets" or "information packets") cause minor, often unnoticed, interruptions of the speech. The major reasons for sending such messages from the base to the mobile radio are to remotely control the mobile radio transmit power and to change the mobile radio frequency channel when required for a handoff.
The mobile transmitter power must be controlled because the mobile transmitter needs to put out more radio frequency power to reach the base station receiver with a strong signal when it is at the outer boundary of a cell than when it is near the base receiver antenna at the center. Lower power is used at or near the center of the cell to prevent excessive interference with other cells. As a mobile moves from the center of a cell to the outer perimeter, the base may command it to increase power in as many as eight steps, designated as Power Levels zero through seven (PL0 through PL7), with PL0 being the maximum.
Consider the station when the mobile approaches the outer perimeter of the cell. The radio signal received at the base station will decrease still more when the mobile set moves further away, and if the mobile transmitter is already set at PL0, the signal level cannot be increased any further by increasing the mobile transmitter power. At this point it is usual to begin a handoff process. First the base system causes a tunable "locating receiver" in each of the six neighboring cell base stations to tune to the mobile transmitter frequency of the mobile which has reached the handoff radio signal strength level. The measurements of radio signal strength in the six neighboring cells are compared, and the strongest one is chosen as the handoff target cell. If there is an available voice channel in that cell, the mobile unit is sent a command message to re-tune to that frequency, and simultaneously the voice connection is switched over to the proper radio channel base unit in the target cell by means of a special central telephone exchange switch which is connected to all the cells and also connected to the public telephone network. This special switch is known as a Mobile Switching Center (MSC) or a Mobile telephone Switching Office (MTSO).
A potential major problem of cellular mobile stations in aircraft in flight is the high level of interference which the mobile set can cause to numerous cells in a city when it operates in an aircraft. This problem and the invention to overcome this problem is explained further below. Another problem is the high level of noise interference received by the mobile airborne receiver.